The present invention relates to a process for selectively etching silicon nitride in the presence of silicon oxide, and more particularly to a process for effectively and efficiently etching a layer of silicon nitride at a high etch rate and with high selectivity with respect to exposed or underlying layers of silicon oxide, particularly in a multilayer semiconductor wafer structure.
The manufacture of multilayer semiconductor wafer structures typically involves the deposition of multiple layers of different materials and patterned etching of selected areas of one or more of those layers on a semiconductor surface. Patterned etching can be accomplished using either masking techniques or layers of materials which are relatively unaffected by the chosen etchant. Etching techniques may be broadly categorized as either wet or dry, with wet techniques using solutions of chemicals, typically acids, and dry techniques using, for example, gaseous ionic plasmas.
Conventional wet etching techniques for silicon nitride (Si.sub.3 N.sub.4) have utilized hot (approximately 145.degree.-180.degree. C.) phosphoric acid (H.sub.3 PO.sub.4) solutions with water, typically 85% phosphoric acid and 15% water (by volume). As the water vaporizes rapidly at this temperature, the etch is performed in a closed reflux container equipped with a cooled surface to condense the vapors. One problem with hot phosphoric acid etches has been that standard photoresist materials used to mask portions of a surface to be protected from the etchant do not stand up well to the high temperatures and aggressive etch rate (50-100 .ANG./min.). Consequently, a layer of a silicon oxide, such as silicon dioxide (SiO.sub.2), has been used to block the etchant from protected areas on the wafer.
As semiconductor technology has advanced, finer geometry patterns are being used to enable higher density structures to be fabricated. Such finer geometries have created additional problems with hot phosphoric acid etchants for removing silicon nitrides due to insufficient selectivity with respect to silicon oxides. That is, while the hot phosphoric acid etchants will attack silicon nitride and remove it much more rapidly than silicon oxide, the oxide is still attacked as well.
Thus, where a relatively thick layer of silicon nitride must be stripped away in the presence of an area of exposed oxide or a relatively thin layer of an underlying oxide, the potential for there to be a deleterious loss of silicon oxide is significant. Nonuniform layer thicknesses created during deposition steps require that overetching must be employed to ensure complete removal of the nitride.
If an underlying silicon oxide layer is thin and the selectivity of the etchant for nitride over oxide is not sufficiently high, and if the etch must stop in the underlying oxide layer, then overetching of the oxide layer can occur. Additionally, for situations where it is desirable or necessary to maintain as much of the silicon oxide layer as possible, an etchant with a higher selectivity for nitride over oxide than is presently possible with hot phosphoric acid etchants is desirable.
Accordingly, there remains a need in the art for a wet etchant process which effectively and efficiently etches silicon nitride at a high etch rate and with high selectivity with respect to exposed or underlying layers of silicon oxide, particularly in a multilayer semiconductor wafer structure.